This is the know "classical" compact unit: all building services are realised in one handy appliance:
Everything is centred on the element of air: air is the medium that transports the heat (on the supply side), air is the heat source of the heat pump (on the exhaust side). Of course, if necessary, the air could be cooled and dehumidified as well, using the same equiment - that might be interesting in hot climates.
Note that only the fresh air required for indoor air quality is used, there is no recirculated air. That is a difference to the systems in wide use e.g. in the US; those use only recirculated air and a far higher air flow rate.
Fireplace, which is the main heating system in a passive house in Friedberg (architect: Blumrich); this automatic oven is also heating domestic hot water; during summer thermal solar collectors top the concept off.
Compact Units available on the market.
Idea #1: Use the fresh air required for indoor air quality also for heating the building
A building occupied by human beings needs fresh air. If the fresh air supply is left to good luck, it should not surprise if indoor air quality (IAQ) worsens.
If on the other hand the heat from the exhaust air is not recovered, there will be significant ventilation heat losses. It is impossible to realise an energy efficient building in this way - especially if the indoor air quality is to be high as well.
Therefore, for energy efficient new construction or refurbishment a mechanical ventilation with heat recovery is really necessary - this at least holds for cold climates (climates, in which one needs a heating system) and in hot climates (climates, in which one needs active summer cooling). For a detailed discussion of this point, have a look at our page on the topic ventilation.
The supply air, which is delivered by the air-to-air heat exchanger, can transport some heat, too. It is not a huge amount, but just 10 W/m² can be delivered to the supply air rooms using the fresh air required for good IAQ (see the calculation on heating capacity of fresh air). That will not be sufficient at all in conventional houses. But in a passive house, the peak heat load requirement is extraordinary low. Indeed, it will be so low that these 10 W/m² available from a fresh air supply will be sufficient (that in fact is the defining condition for a passive house). Thus, some simplified building services systems become possible in passive houses: “Heating with the ventilation system”, without the need for additional ducts or even without the need for a higher duct cross-section dimension. If, in addition, the heater for the supply air is integrated within the ventilation system and the domestic hot water boiler, one ends up with an integrated compact unit:
Heating, ventilation, domestic hot water and cooling (if necessary) can be supplied by just one appliance. Many solutions can be chosen for heat generation: